Differentiate the absence of an element in the environment: define #ENV{default} as a default value when #ENV{activer} does not exist. To do this, the is_null filter allows us to test that #ENV{activer} is not defined. If #ENV{activer} exists but is empty, it will be used. We can thereby differentiate the case of sending an empty value into a form, as shown below when the value sent is that of the "champ_activer_non" input field